1. Field of the Invention
The present invention relates generally to a horizontal machining center, and particularly to a horizontal machining center having a column, a spindle head and a table sliding in X-, Y- and Z-axes, respectively.
2. Description of the Background Art
For a conventional horizontal machining center, for example Japanese Patent Laying-open No. 48-60370 discloses a machine tool equipped with an automatic tool exchange device exchanging tools in a short period of time and also having a simple structure to help its fabrication. The machine tool disclosed in Japanese Patent Laying-open No. 48-60370 has a magazine body adapted to be movable by a magazine driving hydraulic cylinder in a direction parallel to a spindle's centerline. When the spindle has a tool to be exchanged, the spindle head moves to allow the tool to be inserted into an available gripping device of the magazine. When the spindle has the tool disengaged therefrom, the hydraulic cylinder is driven to move the magazine body forward to draw the tool out of the spindle.
Furthermore, Japanese Patent Laying-open No. 58-94951 discloses a machine tool intended to remove a possibility of having swarf adhering on a fitting surface of a tool and increase a table's feed rate. When the machine tool disclosed in Japanese Patent Laying-open No. 58-94951 has a tool to be exchanged, a tool magazine body is placed on a tool magazine support formed at the table and is also clamped to the table. In this condition, the table is moved toward a spindle head to position the magazine body immediately under a spindle. The tool on the spindle is inserted into a tool holder provided at the magazine body and the tool holder descends to draw the tool out of the spindle.
A variety of structures are proposed for an auto tool changer (ATC) for automatically exchanging a tool attached to a spindle with a tool accommodated in a magazine. For example, Japanese Patent Laying-open No. 48-60370 discloses a machine tool allowing a magazine per se to be moved to draw/insert a tool out of/into a spindle. Such a structure entails a driving source for moving the magazine, and the machine tool disclosed in Japanese Patent Laying-open No. 58-94951 allows the magazine to be moved by a driving source that also serves as a driving source for moving a table to achieve a simplified structure and a reduced cost.
In exchanging tools, it is necessary to move the table as far from the spindle as possible to prevent the spindle and a workpiece on the table from interfering with each other. If a horizontal machining center includes a spindle moving on a bed along X- and Y-axes and a table moving along a Z-axis, then, in exchanging tools, the table is positioned in general at a point on the Z-axis that is remotest from the spindle, i.e., the origin of the Z-axis. For this reason, when the table's driving source is utilized to exchange tools, as disclosed in Japanese Patent Laying-open No. 58-94951, it is necessary to move the table further away from the origin of the Z-axis. In doing so, if the table is moved away from the spindle to be farther than the origin of the Z-axis, the table's Z-axis travel must be set to be longer than original, resulting in the horizontal machining center having a bulky structure.